There is high industrial demand for transparent, high-gloss plastic films, e.g. biaxially oriented propylene films or biaxially oriented polyester films. There is also an increasing demand for transparent films of this type which are flame retardant and which have at least one surface layer which is not high-gloss but has a characteristic matt appearance and therefore, for example, is effective for promotional purposes in the construction of exhibition stands or provides the packaging with an appearance which is particularly attractive and therefore effective for promotional purposes.
U.S. Pat. No. 4,399,179 describes a coextruded biaxially oriented polyester film which is composed of a transparent base layer and of at least one matt layer which essentially consists of a certain polyethylene terephthalate copolymer and also comprises inert particles with a diameter of from 0.3 to 20 μm at a concentration of from 3 to 40%. The specific copolymer is a processing aid which reduces the viscosity of the melt comprising the inert particles, thus permitting satisfactory extrusion of that layer. The mattness of the film is achieved by adding the inert particles to the appropriate layer.
EP-A 0 144 978 describes a self-supporting oriented film made from thermoplastic and, on at least one of its two surfaces, bearing a continuous polyester coating which is applied as aqueous dispersion to the film prior to the final stretching step. The polyester coating is composed of a condensation product of various monomers which are capable of forming polyesters, such as isophthalic acid, aliphatic dicarboxylic acids, sulfomonomers, and aliphatic or cycloaliphatic glycols.
DE-A 2346 787 describes a flame-retardant polymer. Besides the polymer, the use of the polymer to produce transparent films and fibers is also described. When films were produced from this the phospholane-modified polymer the following shortcomings were apparent:
The polymer is very susceptible to hydrolysis and has accordingly to be very effectively predried. When the polymer is dried with commercial dryers it cakes, however, and production of a film is therefore possible only under very difficult conditions. The films produced, under extreme and uneconomic conditions, also embrittle on exposure to heat. The deterioration in mechanical properties is so severe as to make the film unusable. This embrittlement occurs after as little as 48 hours of exposure to heat.
The prior art gives no indication as to how at least one surface of a film can be given low gloss while retaining high film transparency, where the film is also flame-retardant to DIN 4102 and does not embrittle after exposure to heat.
It was an object of the present invention, therefore, to provide a polyester film which has at least one matt surface, where this is flame-retardant to DIN 4102, and is simple and cost-effective to produce, and has the good physical properties of the known films, does not pose problems of disposal, and does not embrittle after exposure to heat.